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Countries that have unmanned and manned fighter teaming projects
I have been wondering what countries currently have military programs related to teams of manned and unmanned fighter aircraft? For instance, I know that the US has several programs related to this such as the Have Raider, which was a demo, or more newer programs such as skyborg, digital century series, or more prominently the loyal wingman project. However, I am not sure about other countries.
I appreciate any additional info on this.
Many thanks!

Welcome.
Sorry, but I am inclined to close this. To answer it would be required to compile a list that would need constant work to be up-to-date. No single answer would suffice, unless we force people to only put their efforts in a single community wiki.

Who says that any answer here MUST be constantly kept up to date?

@MichaelHall what good is to the community an outdated answer?

@Federico, questions and answers are dated, information changes.  There are probably more than a few stale ones out there right now.  Any question or answer may be updated of course, but why shut one down because some information might change later, and it might not be updated as rapidly as you desire?  Just give it a chance man!

@MichaelHall it is not about giving it a chance, it is about keeping the site healthy. allowing mindless list questions is not productive in that sense. having outdated replies is also counterproductive.

Information accurate as of a certain date IS useful from a historical perspective.  After 1969 we didn’t purge all libraries of all references to a possible future moon landing. List questions is a different issue.  Glad that you just voted to close though, and restrained your moderator hammer.

https://meta.stackexchange.com/questions/158809/why-was-my-shopping-list-question-closed - this is our equivalent of a "shopping list" question, clearly its not asking to go shopping but it needs to be closed for the same reasons

@MichaelHall you have also just described why printed books have their limitations, and we are not limited in the same way.

@Federico seems like a valid question to me. There's only so many projects of such kind and only so many will begin in future. There's plenty of other questions that could use moderation.

@Mridul made my point.  I'm fine with following the rules here, it is the inconsistent application that is irksome.  Thoughtful, interesting and well written questions are often closed for some minor excursion from the guidelines, while very poor quality ones remain open.

@Jamiec, yes, I completely agree!  But closing a question before it is answered because it might require a future edit is attaching a limitation that wouldn't otherwise exist.

How about we edit to "what countries currently have..." so it's anchored in time and won't "require" updating if new projects start or old ones end years or decades in the future? Having approved a different edit, I can't edit that in, but maybe somebody else could. @MichaelHall - that work for you?

I did not expect my mindless question would ignite such a discussion. I apologize for the problems associated with the problem. So, to confirm my understanding, to find the answer to such questions, either do the research on your own (which I have already done), create a wiki page and hope that others would contribute to it, or post it on reddit which has admittedly much lower quality of reply? Although as @Mridul mentioned, not many projects of interest exist in the first place.

@RalphJ, I had no objections before.  Because each question and answer is date stamped it should be easy to discern the relevance and accuracy at some future date.  In other words, I’m not the one who needs convincing!

Adding a date doesnt solve the problem! If we take it to it's absurd end we end up with "Countries which have ...... august 2023", "countries which have ..... feb 2027" ad infinitum. List questions do not fit well in our format. They never have.

In the UK, the Tempest aircraft is planned to enter service in 2035:

The aircraft will support scalable autonomy, which will enable manned,
unmanned and optionally manned operations.

According to Wikipedia, it's a "sixth generation" aircraft being developed by the "Team Tempest" consortium which includes the Ministry of Defence, BAE Systems, Rolls-Royce, Leonardo UK, and MBDA UK.  Italy, Sweden and Japan have shown interest in co-operating in the development of some parts of the system.
There is also a French/German/Spanish programme called the Future Air Combat System:

The FCAS will consist of a Next-Generation Weapon System (NGWS) as
well as other air assets in the future operational battlespace.
The NGWS's components will be remote carrier vehicles (swarming
drones) as well as a New Generation Fighter (NGF) - a sixth-generation
jet fighter that by around 2040 will replace current France's
Rafales, Germany's Typhoons and Spain's EF-18 Hornets.

Turkey is developing TAI TF-X and the two-seat variant will use:

the MUM-T (Manned Unmanned Teaming) capability of the TF-X with
maximum efficiency. With the additional seat, the pilot in the back
will be able to coordinate and manage drones like the Bayraktar
Kizilelma & Bayraktar TB2.

An unmanned version of the proposed Russian PAK DP fighter is under consideration.


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Descent of Light attack aircraft to loiter altitude
How do Light Attack Aircraft (LAA) such as the Embraer Super Tucano descend from their cruise altitude to their loiter altitude to be on-station above ground troops? I want to know the way these airplanes descend and what are typical rate of descent and descent gradient. Do they descend steeply and accelerate?
I know that commercial airplanes descend at a 3 degree angle. Do the LAA also descend at this angle?

Commercial airplane approach and land at 3 degree. Descend is not necessary 3 degree.

I'm not an expert on this particular subject but commonly any manoeuvre is Executed in the most fuel efficient manner unless operational conditions require otherwise.

I cannot answer for the Supertocano, or even light attack aircraft, but I can tell you that in a tactical environment nobody cares about your rate of descent or descent angle, only that you are in your assigned block altitude at the appropriate time.
Descents aren't planned or adhered to in same way that airline descents are, and fighter type aircraft aren’t controlled in the same manner.  There is no need to be smooth for paying passengers either - As long as airspeed and G restrictions for your type aircraft are followed, and you don’t violate any theater safety rules, (like blowing through another airplane’s block altitude while they are on station) the area commander just needs you to be on-station, on-target, on-time.
One day you might make an easy cruise descent, the next you might do a split-S.  The threat environment plays a part as well.  You may have the luxury of making a long easy fuel efficient descent, or you may need to stay high to avoid ground threats, then expedite your descent overhead your station.
I used to like to make a slow descending barrel roll because it was a good way to lose a lot of altitude quickly while visually clearing the airspace 360 degrees around me.

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What are the typical max speeds & g loads at which internal weapon bay(IWB) door can be opened for weapon deployment?
What are the typical numbers for popular aircraft like the F-22 & F-35 which have IWB?

I think it depends of the aircraft. Both mentioned fighters may have different values. If i were the guy making specifications, i would have asked for the bay to be openable for the whole flight envelope (up to max speed at 9G)

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What are the unique sounds emitted by Embraer E190-E2 on final?
I live close to an airport where these aircraft frequently land. When the E2 is between 2-3 thousand feet, something which I can only describe as whale moans, is heard.
I say they are unique because I can’t hear them from other aircraft. However, this airport seldom get heavy traffic, so maybe this sound is common among heavier aircraft—I wouldn’t know.

This was answered by an pilot acquaintance of mine:
The most recent generation of engines on the E2 comes with a geared turbofan. In the final stages of descent, the engines rotate at speeds which causes resonance in the combustion chamber. The distinct sound is that vibration.
This sound can also be heard when departing the gate, when the engine experiences similar rotational speeds.
Mitigation of this requires a redesign of the combustion chamber.

How nice of you to come back and share the answer to your own question 2 years later! +1, glad you figured it out.

It is even "official" now, see Schiphol website


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Why is an aircraft carrier's arresting cable made from steel?
Steel seems to be what modern aircraft carrier arresting cables are made from.
It's strong, provides some elasticity, and serves its purpose well. However, it does need to be changed after every so many traps due to metal fatigue (about every 125 landings from what I've heard).
Why not use a more modern material such as Kevlar? Wouldn't a material such as Kevlar reduce the frequency which the cable must be changed? It's lighter, more flexible, more durable, can tolerate more temperature extremes, is not susceptible to corrosion, and it might even be cheaper - since it's made from nylon threads. It also has the added advantage of being less dangerous in the rare event of a cable break under load.
This BBC Documentary (no longer available) posed the same question, but didn't really provide any answer.
Why are modern aircraft carrier arresting cables still made from steel?

I'm pretty sure Kevlar in a wire form has about 10% less capacity than steel of the same size. The other issue is that Kevlar is not more durable, ask a cop if a Kevlar vest will stop a knife attack...

@RonBeyer Both of your claims seem to be refuted in the documentary. Kevlar stops a knife (it depends on the weave selected), and has at least at much tensile strength as the same thickness of steel cord, although they say they expect a little more.

Kevlar alone will not stop a knife attack, it needs a "stab resistant" material as well, like a chain-mail overlay or very thick panels. And as @ymb1 said, once Kevlar stretches, it doesn't rebound like steel, you could say it is much more brittle. Most Kevlar vests are junk after a single bullet, and if you hit the same spot twice, it would not stop the bullet.

@ymb1 In a vest it sure does - that's really the result of a sharp object attempting to go through a flat surface. A cable is a bit different in structure and behavior. (ex: a steel plate will deform after being shot as well)

@RonBeyer You don't seem to be correct. Stab Vests are made from Kevlar, and it's the weave of the material and layers that stop the knife from penatrating: https://crimefictionbook.com/2015/06/17/can-a-bulletproof-vest-also-stop-a-knife/ Stabbing Kevlar is also tested in the same documentary (it's just a sheet of Kevlar laid over a hole and stabbed repeatedly with a thin pointy knife, like a shiv).

@ymb1 We're discussing steel cable, not 6000 alloy cable. Plates and flat surfaces will have very different properties than a cable. Can we stay on topic?

Look fellas, if you think that's the answer, please put it as an answer below. We'll let the community vote and decide if it's a good argument or not.

Kevlar is a non-starter. It decays in UV light. Steel can be exposed to sunlight forever and stay the same. Kevlar cannot.

Also friction, wearing, and lubrication is a important concern, too.

The single most important thing for an arrestor cable is that it works reliably, compared to this incremental differences in weight or durability are fairly minor considerations.
One of the major advantages of kevlar is that it has a very high strength to weight ratio, however all this really achieves in this context is that the cable is thinner and lighter and I would guess that the weight of spare arrestor cables is vanishingly trivial in the context of the total payload of an aircraft carrier.
Another consideration is that Kevlar has much lower stiffness than steel, which will be further exacerbated by the differences between a fibre rope and a braided steel cable. This means that it will extend further under the same load. Not only does this mean that it allows the aircraft to travel further before it stops but you also need to fundamentally redesign the whole arrestor system to account for the very different material properties of the wire and then the new design would need to be tested, evaluated and certified.
Also, while kevlar is considered abrasion resistant by the standards of textiles it is nowhere near as resistant to abrasion as high strength steel cable. Consider that industrial steel cables may have a design life in the order of decades (think of suspension bridges) whereas high strength polymer climbing ropes are typically retired after a few years, mostly because of the cumulative effects  of abrasion, dirt and UV exposure. Also steel is almost entirely unaffected by contamination by spilled fuel, solvents, lubricants, hydraulic fluid etc.
Similarly the arrestor hooks on aircraft are made form high strength steel, which is much harder than Kevlar so even a small burr or sharp edge (as you miht get form landing or combat damage) could cut through a kevler rope like knife whereas a steel cable is much more resistant to sharp edged shearing.
125 loading cycles is nowhere near where metal fatigue would be an issue, especially in steel, so it seem more likely that the life of the cable is based on direct wear and local overloading from the arrestor hook. Steel being relatively hard while having reasonable ductility  is much better at tolerating this sort of wear and tear than most other materials.
It's also worth noting that overloading beyond the yield stress (but before total failure)  will often produce  a noticeable kink in a steel cable whcih is a warning sign that it shod be replaced. This gradual failure mode (ie damage is measurable before it becomes critical( is one of the major reasons why steel is preferred for safety critical structures ie you get some waring of impending failure.
However the most compelling answer is that steel cables are proven to work well for the job and so there is no compelling reason to redesign the whole system to use a new material for the sake of fairly marginal (if any) benefits in performance.
It's also worth noting that textiles are use for the arrestor nets for emergency landings where it is known or suspected that the landing gear on a particular aircraft is compromised. Here the light weight and greater stretch is a distinct advantage as it mitigates potential damage to the air-frame.

I guess a lighter cable would be safer for ground crew though, in case it does fail?

The short answer is the cables are made from steel because that design has been tested and found suitable for the purpose.
The military, and aviation in general, tends to keep using a working solution once we have one because designing new solutions is expensive and time-consuming.

From a materials science perspective there's no reason you couldn't make an arresting cable out of Kevlar: It has adequate tensile strength to be used in this application. To do so however you would have to perform the same kind of fatigue testing on it that was done on the steel cables, determine its useful service life, and then add an appropriate margin of safety after which it's removed from service so the cable doesn't snap and send a very expensive fighter jet careening off the end of the deck (to say nothing of whipping the pendant around to slice deck personnel in half).
You would also have to consider wear - breakage of the individual strands, damage from friction, grit, contamination from oil/fuel/other materials, etc. - and its effects on Kevlar, or any other proposed replacement material.
It's possible, after all that testing, that Kevlar would be found unsuitable due to a short fatigue life (more downtime of the deck while changing cables), higher per-unit cost, or any number of other reasons.

Note that there is at least one patent for a hybrid-type arresting cable with a composite (Kevlar or Nylon) core and a steel outer wrap. We may see it used in next-generation arresting gear, or the numbers may get crunched and the "traditional" steel cable found to be a better solution when all factors are considered.

This seems to also provide some good data: http://www.christinedemerchant.com/rope_material_aramid.html

Each of those sites only tells part of the story. Ultimately a single thin strand of Kevlar will handle more load before failure than steel, but its performance under shock loads is inferior to steel, and once it begins to fail (deformation) it is more prone to fracture (breakage). When combined into a rope however Kevlar's performance improves because a polymer rope can be drawn from thinner material and have many more strands in a given thickness than a steel cable. It cannot handle as many internal failures as a steel cable though (load distribution is compromised & it fails under shock).

Great analysis. I feel adding it to your answer would make your answer pretty complete and acceptable as "the" answer. :)

@SnakeDoc That analysis is, in itself, woefully incomplete - there's a lot more to it: Temperature stability, reactivity (especially on an aircraft carrier in salt water), etc. -- that's why I didn't include it in the first place. The design of arresting gear cables could be a semester of Materials Science class in and of itself :)

Its a good question.  The answer why is part technology, part economics, part life cycle reality and part bureaucracy.  In short, it may be possible to build arrester pendants out of other materials than hemp cored steel rope, but life is never that simple.
First off is reliability.  For the entire history of CATOBAR aircraft carriers in naval aviation, the ships have relied on arresting gear using steel rope cross deck pendants to arrest landing aircraft.  With 100 years and literally millions of traps under its belt and few, if any, reliability, cost or maintenance issues, there exists no impetus to change from that design.
But for a moment, lets assume we wanted to design a new cross deck pendant making use of, say, a kevlar rope or more exotic materials eg woven carbon nanotubules, etc.
LIFE CYCLE CONCERNS

Inception:  Is there a pressing reason that offers enough benefits to change away from the existing steel rope technology to a more modern alternative?  Is there an increase in reliability, cost savings, safety, etc. that make it worth spending a tremendous amount of time and money to improve the existing solution.
Development:  Requires time and money to develop a new cross deck pendant alternative.  Can it do all of the things the current steel rope can?  Is it flexible?  Does it interfere with existing carrier infrastructure?  Are there drawbacks to the new design such as a increase brittleness or a need for high maintenance to work properly?  Does it offer a long shelf life in harsh environemnts such as sea spray, etc and still work properly?
Manufacturing: Does the new design require expensive and exotic raw materials?  Is it more difficult to manufacture?  Or does it require a much more stringent manufacturing environment or elaborate quality controls which are more difficult to realize and present operational hazards if not met as opposed to manufacture from steel rope?  Does it require the use of toxic chemicals which are hazardous to personnel in the plant or present an environmental hazard to dispose of properly?
Service: This one can open up a host of problems which are never fully anticipated and can often show up years or even decades after the product is introduced to the fleet.  This can come in the form of difficulties transporting, stowage aboard ship, degradation during use, unanticipated structural failures for the reasons listed above, or operational hazards to the flight deck crews, aircraft of the ship.  A new design like this would undergo years of testing, often wringing out these kinds of problems, but it can result in an issue which makes further development unwarranted.
Disposal: Inevitably the pendant must be retired and subject to destruction.  Can it be recycled?  If not, does it have to be shredded and incinerated?  Or buried in a landfill?  Does this process create toxic chemicals which are hazardous to humans or the environment?

COSTS
A new cross deck pendant design is going to cost millions and this expenditure is really unwarranted if the only reason for doing so is that other, more exotic materials exist which are stronger.
BUREAUCRACY
The Navy is, by far, the most stodgy and difficult to convince of unorthodox technology of any of the branches of the military.  They often don't know what they want and this dramatically compounds development times and costs for procurement of weapon systems.
Consequently, it appears that hemp cored steel rope will be used as arrester pendants for decades to come.  It is possible that other endeavors will make use of exotic material ropes which, once they have been developed through a mature life cycle will have proven more useful for the Navy to evaluate for this purpose.  A similar thing happened with the Electromagnetic Aircraft Launch System (EMALS) which the Navy is developing as a replacement for the steam catapults currently used on its CATOBAR carriers from an existing linear motor technology which has been used in amusement parks for over a decade now.  But even this system has been fraught with development problems and does not quickly translate over to use aboard an aircraft carrier.

Many of the "benefits" you listed just don't matter:

It's lighter, more flexible, more durable, can tolerate more temperature extremes, is not susceptible to corrosion, and it might even be cheaper - since it's made from nylon threads. It also has the added advantage of being less dangerous in the rare event of a cable break under load.

Lighter:  This is carried on an aircraft carrier, a few thousand pounds more or less is no big deal
More flexible: Steel is flexible enough, being more flexible is optimizing something that does not need optimization.
More Durable:  Maybe a mildly redeeming quality, but Kevlar weaves can get small metal particles trapped in them which cause long term abrasion and weaken the cable.
Temperature extremes:  Arresting cables are in the same environment as humans:  roughly 0°F to 120°F.  Steel and Kevlar are both fine until around 300°F.
Corrosion resistance: Steel is just fine against seawater, jet fuel, and common solvents.  Kevlar is slightly susceptible to abrasion from small metal and dirt particles getting in the weave.
Cheaper: When it comes to jet planes and military operations, small cost differences are not a consideration.

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Has a commercial aircraft ever taken off with its landing gear up?
In the extremely scientifically accurate movie Airplane the Autopilot takes off with the gear up.
Has this ever happened? Could any commercial or 4+ passenger GA aircraft do this, say with a 1%+ chance of not crashing?
Maybe with a smooth, small ditch that would guide the body and/or grass it could doable. The belly would certainly be warm! (Or on fire)
Silly question, but didn't find anything about it online.

Yes sure, any flying boat does :)

I don't have any authoritative evidence on this, but I expect the friction of the ground would prevent the aircraft reaching takeoff speed. There might also be some difficulties in raising the nose.

@ThatCoolCoder not only that, but most airplanes also need to rotate (raise the nose to increase angle of attack and therefore lift) on take off, and wouldn't be able to do it without the gear.

Ski equipped LC-130's take off this way from arctic & antarctic snow packs... wheels down to operate on conventional runways, wheels up to operate on the snow. If I have time to find a good photo, I'll post as an answer, although anyone else is welcome to as well.

@RalphJ, I would could the skis as (a special kind of) landing gear though.

@JanHudec  If the skis are all that you've got (like photo in Mike Sowsun's post, or a pure airplane-on-skis), I'd entirely agree.  On an LC-130, the wheels are still there, but they are extended/retracted thru the skis, so the case could be made that it's still a "gear up" takeoff -- especially since the skis can't be retracted.  But this is an exercise in terminology more than in what the OP was asking about, so I'm not inclined to pursue this thought much further.

@RalphJ that's just two alternative landing gears, with the LC-130 being unusual in that the skis, when equipped, are not retractable (on most aircraft when skis are mounted, they move with the gear).

Thinking about it: It can only be on the belly if it has before landed with the gear up, right? (If we exclude experiments or technical verification.) It would most likely not be in flight worthy condition after that, surely not nominally. The only conceivable scenario for a gear-up start would be after an emergency landing in a remote place without the option to repair or leave without the plane.

the Autopilot  Get it right.  That's Otto Pilot.  :-D

Excluding planes that use something else than landing gear for take-off—seaplanes, using launch rails, or air-launched—there was one accident where a plane went around after landing gear up by mistake: Crash: PIA A320 at Karachi on May 22nd 2020, impacted residential area during final approach, both engines failed as result of a gear up touchdown. Two points to take from that accident:

The plane was only able to lift off again because it still had way higher than normal take-off speed. Since sitting on the engines does not allow as high pitch attitude as normal rotation on gear, the angle of attack is lower and so higher speed is needed to get the same lift.
The damage to the engines caused during the event caused them to both fail in a minute or so.

That shows it's unlikely it would be possible to take off this way from stand-still. Of course this does not show no plane could do that, but most commercial aircraft are similar in that regard. They are designed stay intact enough on a gear-up landing so the people on board won't be injured, but will be damaged enough that they won't be able to take off without repairs.
Side note: the autopilot can't start the engines on its own and the pilots would hopefully not be insane enough to do that with airplane sitting on its belly.

Comments on that link mention "a Tu-154 touching down with landing gear retracted, skidding a solid 650m on the runway while the engines spooled up, lifting off again for another approach and landing successfully."  Another commenter points out that its tail-mounted engines will keep them out of harms way while scraping the belly, unlike the A320 accident aircraft.  So this idea is a non-starter for an A320, but maybe not for other designs.

@PeterCordes … thanks; fortunately there is even a reference: Malév flight 262. While the tail-mounted engines would not be damaged, it really surprises me the plane was able to lift off again, because the bottom of the aircraft is flat so without gear it cannot raise the nose to get enough lift.

The fuselage curves upward some distance ahead of the elevators; maybe at the speed they were going (with engines already spooling up before touchdown) that short lever arm was enough for the pitch authority the elevators could provide?  (Compared to how far the center of mass was ahead of the centre of lift from the wings, which is what the tail would have to balance on the other end of that lever.)

Just a raw estimation to be added to the other answers:
At takeoff, engine's thrust is used to accelerate the airplane and to overcome aerodynamic drag plus rolling resistance of the landing gears. Or, in the case of our sliding airplane, the sliding friction.
In order to understand if making the airplane slide on its belly instead of rolling on its tires is doable, let's ballpark estimate these four terms (acceleration, aerodynamic drag, rolling resistance and sliding friction). No worries, just a couple of simple multiplications.

Acceleration: since the airplane has to reach the same takeoff speed within the same airfield's length both with and without landing gears, then it has to accelerate in the same way for both cases; from basic physics, acceleration is $a=V²/2L$  and for Newton's second law the relevant force to be applied is $F=ma=mV²/2L$ where $m$ is the airplane's mass, $V$ the takeoff speed and $L$ the field's length. Let's use a B747 to do the maths (SI units):
$F=(370'000\times85²)/(2\times3'300)=405kN$

Drag: let's evaluate it just before takeoff which is where the highest value is reached before (any kind of) landing gears become useless; we use the usual drag equation $D=½\rho V²SC_d$ at sea level with $C_d=0.02$ as given in the B747's polar:
$D=½\times1.125 \times85² \times511 \times0.02=40kN$

Rolling resistance; this is simply the weight times a coefficient which depends on the materials; for tire on asphalt this is some 0.03:
$F_{roll}=370'000 \times 0.03=11kN$

Sliding friction; also in this case it is simply the weight times a friction coefficient that for steel (I couldn't find aluminium) on asphalt is more or less 1:
$F_{slide}=370'000 \times 1=370kN$

So, let's do the sum to get the needed thrust $T$ for the case with landing gears (1.+2.+3.) and without (1.+2.+4.) respectively:
$T=405+40+11=456kN$
vs.
$T=405+40+370=815kN$
The latter represents an increase of some 80% in respect to a takeoff with landing gears... most probably not feasible.

Wing moving through the air needs to be pushed with a force that is typically between 0.05 and 0.10 of the weight it is carrying. A bit more at the high angle of attack occurring at take-off. Since commercial aircraft have to be able to continue flying with one engine inoperative, with all engines it needs at least 0.2, but usually around 0.3 of its weight. If the coefficient of friction of aluminum on tarmac was just 0.1, the engines would have plenty of power to accelerate it. I don't think it's that low at the weights involved though.

… remember, we are not talking about asphalt and aluminum ski sliding on it, we are talking about tarmac (asphalt-glued gravel) and normal forces under which the aluminum piece is abraded rapidly.

@JanHudec: you're definitely right, I was just considering the very beginning of the takeoff. I'm going to update my answer with a more general approach.

@JanHudec: done!

The ground friction will reduce some with speed as the wings take some of the weight, if the plane has enough thrust to overcome static friction.  Maybe some high power/thrust-to-weight craft like a learjet?  Or a turboprop?  https://en.wikipedia.org/wiki/Thrust-to-weight_ratio#Aircraft_2 shows Concorde with a TWR of 0.372 (full fuel, full afterburners), and a Tupolev 154B at 0.360.  vs. a Boeing 747-8 at 0.269 (MTOW, full power).  None of the wiki examples are turboprops (which IIUC have good low-speed thrust, and might have lower takeoff speed which would require less F x v = friction power)

@PeterCordes: yes, off the top of my head I'd say a 10% on MTOW is taken from lift, but then we have to add the relevant induced drag.

Now, that could change a lot if we're looking at an empty 747, 757, or 777 (or whatever), with say 1 hour of fuel and an empty cargo bay (no seats or anything). https://www.airliners.net/forum/viewtopic.php?t=747169  . (Concorde is a good choice for thrust-to-weight, but its high takeoff speed makes it a terrible choice for this.)

@PeterCordes: well, actually a B747 has some 1'000kN of thrust, so it might be able to roll anyway. But then you have obviously to forget OEI takeoff-roll from hot&high airport. And obviously the rotation at the end of the airfield would be impossible ;)

Would there also be a reduction of lift with the wings that much closer to the ground?

@WGroleau: a reduction in induced drag aka drag due to lift

If that is a "raw estimation" I'd hate to see a detailed one!  Nice!

@MarkStewart: there would have been a lot of integrals

Amphibious aircraft. These only have the  gear down when taking off from the ground, not from the water. For instance, de Havilland Canada Twin Otter can be configured as amphibious, it was used by multiple airlines and with 19-20 passengers it is quite an airliner.
Rocket launched (Space Shuttle) or air dropped (X-15) aircraft also  use the landing gear only for landing but the most known examples are not commercial. SpaceShipTwo however seems commercial at least in the plans.
While the unprepared land base aircraft may have enough engine power  at least on ice, I cannot imagine how it could maintain the necessary orientation without the landing gear while taking off.

Float planes and flying boats have taken off from snow or ice in the past.
Here are two Videos of a Cessna float plane taking off from a frozen lake.
Cessna 185 taking off on the ice with floats - Red Lake, Ontario
Off the ice on floats, Red Lake

Aside from the other answers, I think what you’re asking is whether an aircraft could or has taken off where undercarriage would normally be extended.  I can’t say that this has never happened but it seems most unlikely.  Propeller-driven aircraft would typically have the prop touch the ground with the wheels retracted, but that wouldn’t be the case for a jet, which would likely have a high enough thrust/weight ratio to overcome friction, especially on a wet runway.  That raises the question of why any pilot would attempt this; if the undercarriage was inoperative then they would be faced with a wheels-up landing.  If they were attempting to steal a plane then why not choose one with its undercarriage extended.  There are more unlikely scenarios but I don’t need to list them here.  The only definitive answer would be from someone who has seen/executed this manoeuvre, if it’s never happened then it’s unanswerable.

I don't think any plane could take off like that, but one went around like that—and crashed due to the damage it caused.

Note that wheels-up landing is not a huge deal—all planes are designed so that if they land wheel-up (on runway or other sufficiently flat ground), they remain sufficiently intact to avoid injury for the people on board.

May happen if there is some danger on the ground so grave that attempting to take off with such an aircraft becomes an option. Like enemy advancing, for instance.

@Jan Hudec - a 737 has a thrust/weight ratio of about 0.3 so it would struggle to move on a dry surface but may do better on a wet runway.  Once it started moving the left from the wings would reduce friction from the ground.

@h22: the problem is that if the plane has engines low, they will be damaged and won't work for long (as happened in case of PK-8303) while if the engines are high the airplane sits on the fuselage and has no way to rotate and thus increase lift to actually take off.

The most likely case for a commercial airplane taking off "gear up" would be a large floatplane (eg. a float-equipped Twin Otter) taking off from a grass runway on the floats.

@h22 You can always beg an advancing army for mercy, physics rarely responds well to that and there are cleaner ways to kill yourself if that's the goal.

Although the thing sounds extremely perilous, the argument that it just might work does get a boost from ground effect.

Malév Flight 262 performed a gears-up touchdown, skidded on the runway for 650 meters, then took off again and flew for 16 minutes before making a second landing.
https://en.wikipedia.org/wiki/Mal%C3%A9v_Flight_262
This was probably possible due to the Tu-154 landing gear pods, into which the wheels retract during flight, which acted as sleds and shielded the landing gear, wing and flaps, as well as the Tu-154's tail-mounted trijet engine configuration and high thrust-to-weight ratio.


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Incidents of V1 flying bombs being nudged instead of shot down
There are stories regarding V1 flying bombs being nudged off course that I have heard numerous times over the last 50 years. I can understand the reluctance to shoot a bomb from directly behind it but I really wonder how often a V1 was truly nudged. Does anyone have any evidence that this was a common occurrence or was it something that happened once and the story was repeated ad infinitum?
Please note that I am not questioning whether this strategy ever occurred but the frequency/commonality of it occurring.

Probably not often. Catching up with the V-1 in level flight was not easy for the (piston-powered) aircraft of the time. Quotes from Osprey book "Meteor I vs V-1 Flying Bomb".

[L]egendary British test pilot
Capt Eric Brown described the performance of the [Meteor] Mk I as ‘pedestrian’. Compared
with the high performance piston-engined fighters then in service with the RAF (the
Tempest V and Spitfire XIV), the Meteor offered little in the way of superior
performance. Where it excelled, however, was at low level – exactly where the V1
operated. The Meteor I was faster than any of its contemporaries at such altitudes.
This was just as well, for the V1 boasted an average speed of roughly 400mph between
1,000ft and 3,000ft. At those heights the Tempest V and Spitfire XIV could make
405mph and 396mph, respectively, using 150-octane fuel. The Meteor, on the other
hand, had a top speed of 410mph at sea level.

The few experimental Meteors were pressed into service for that reason.
Most fighter attacks on the V-1 were diving attacks, even with the Meteor.

While the first V1 to be brought down by a Meteor was
not shot down by cannon fire, the remaining 11 credited to
No. 616 Sqn were, using the Meteor I’s quartet of nose-
mounted 20mm cannons. In order to negate the missile’s
high speed, a pair of jets would climb to an altitude well
above the V1’s average height of 1,000–3,000ft. Once the
incoming weapon had survived the hail of flak from the
guns located on the coast it was the Meteor’s turn. Vectored
by radar, the pilot would then get into the right position and
altitude, open the throttles and begin scanning the terrain
below. Once the V1 was spotted, he would trade height for
speed. Diving at speeds in excess of 400mph, the pilot
would position his Meteor above and astern of the V1.
Because the Meteor’s guns were mounted in the nose
and not the wings, the pilot did not have to worry about
converging fire. Engaging the missile at long-range once he
had the V1 in his state-of-the-art Ferranti Mk II gyroscopic
gunsight, pilots routinely registered hits from 700 yards
astern, but in most cases the damage was done at the more
lethal range of 300–100 yards.

There was one tipping attack by a Meteor (too) on August 4:

This event occurred on 4 August 1944 when Flg Off T. D. Dean (flying
EE216/YQ-E) used the wingtip of his Meteor to flip a V1 on to its back, sending the
missile diving to earth.

He did it that way because his cannons had jammed, which was not an uncommon occurrence on the early Meteors.

I half expected the guns to jam because several other pilots had experienced that problem
before me. I also knew that any sudden movement would upset the V1, and so when
my guns failed I already had a good idea of what I should do. I just followed on in and
tipped it up. When I got back to base I found that there was a small dent in my wingtip
where I had hit the flying bomb. This was the only damage to the aircraft, which was
serviceable again after a few hours.

A similar attempt had been made by a Meteor on July 27, and again due to similar jamming, but on that occasion the approach was called off due to proximity of balloons.

Sqn Ldr Watts (EE213/YQ-A) was unfortunate in having trouble with his
guns as he was about to open the squadron’s Diver score near Ashford. Flg Off
Dean (EE215/YQ-C) sighted one Diver and followed in line astern at
405mph. He had closed to within 1,000 yards of the bomb, estimated to be
flying at 390mph, when he was turned back by control owing to the proximity
of balloons. Pilots are convinced that given favourable weather and good plots
nothing can prevent the Meteor knocking down the latest Axis weapon.

Given all these factors, I'd venture a guess that less than 1% of the V-1 kills were by tipping.
Over a comparable on month period in the summer of 1944, there were over 750 V-1s claimed by British fighters. Most (498) were shot down by Tempests (0.21 kills per sortie), followed by Spitfires (227 kills; 0.10 kills per sortie.) The Meteors were only credited with 12.5 kills (one shared with a Mustang), and a rather lower ratio of 0.046 kills per sortie. Some of these failures were attributed to gun malfunctions or being interrupted / targeted by allied AA or aircraft, whom mistaken them for German (jet) fighters etc.
Overall in 1944 some 1,902 V-1s were claimed by British fighters (and some 40 by Americans). So the tipping attacks, while spectacular, have to be kept in that perspective.

There's also one USAAF pilot which claimed such wingtip victory. As related in Osprey's V1 Flying Bomb Aces:

USAAF pilots also continued to play a small part in anti-‘Diver’ operations
until their units’ eventual move to France. Among them was eight-victory
ace Maj Richard E Turner, the CO of Mustang-equipped 356th FS/354th
FG. On the evening of 18 June he flew along the coast near Hastings
looking for V1s, and at 2100 hrs he found one;
‘I sighted one below and dived on it, pulling out behind it, but slightly
out of range. I tried to close the distance, but the missile was just a little
too fast. I chased the infernal machine for ten minutes, alternatively diving
to gain speed and pulling up to lob long-range bursts at it. Eventually one
of my bullets must have scored a chance hit in the engine, for suddenly
it emitted a long streamer of yellow flame and quickly lost speed. In a
curving dive, it plunged into a vacant field below, where the missile
exploded harmlessly.
‘Encouraged by my success, I proceeded back to the Channel area to pick
up another. I began to wonder how I was going to get the next V1 because
most of my ammo was expended and my gun barrels had burnt out. Soon
I saw another one and made a very steep dive in order to gain extra
overtaking speed. This bomb must have been moving more slowly than the
first one, for I almost overran it as I pulled out of my dive. As I flew
alongside the little monster, I had a new idea. I knew they were controlled
by a gyro-guidance “brain”, and perhaps this mechanism could be upset
without gunfire. I carefully edged closer to it and placed my wingtip about
a foot under its tiny fin. Rolling my aeroplane suddenly neatly flipped the
V1 upside-down, and it promptly spun into the shallows of the Channel
near the shoreline, where it blew a useless hole in the water. Jubilant with
my success, I rushed back to Maidstone and hastened to tell the other pilots
of the new pastime that I had discovered.’

There was another USAAF pilot which claimed a similar feat:

2Lt Melvyn Paisley of the P-47D-equipped
390th FS/366th FG, shot down the last V1 to be claimed by the USAAF.
Participating in his unit’s first mission from Asch, in Belgium, Paisley had
become separated in cloud from his section. He later recalled;
‘As I broke out of the cloud cover an object caught my eye at “eight
o’clock”. It was low, maybe 2000 ft below me, steady on course and black
as night. With a brisk “phut . . . phut . . . phut”, the Jerry V1 buzz bomb
sped on its course to England or maybe Antwerp. I shoved the throttle to
the wall and went into a shallow dive, following close behind for 50 miles,
plotting its course on my kneepad map. It was on a line towards Brussels.
‘Surveying the area, I noticed there was mostly farmland ahead of the
V1. I switched on the water injection and moved up on the V1, carefully
placing my wing under its wingtip. The Doodlebug was much steadier
than my “Jug”, probably owing to the absence of a nervous pilot flying it.
With a prodding right roll of my aileron, my “Jug” tipped the V1’s right
wing into the sky. Within seconds the V1 lost its brains and tumbled
downward. Rolling back, I took a short blast at the black body as it fell,
hoping to get a little camera coverage. What a stupid move that was. My
plane lurched upwards as the V1 burst into the open field below – I barely
escaped the explosion, which sent hoards of mud spurting into the air.’

I'm not sure if any Tempest pilots managed that. There's a brief account of 3 failed attempts by Canadian Plt. Off. David Ness of No 56 Sqn (which had just converted to Tempests).

One of
No 56 Sqn’s Canadians, Plt Off David Ness, was less successful, however,
as having used up all his ammunition he then tried three times to turn
the V1 over with his wingtip.

It's not exactly spelled out why he failed.
WO Tadeusz Szymanski of the USAAF (flying a Mustang) however related in more detail that some V1 were apparently somewhat resistant to tipping:

‘The thing was jerking along and the elevator was flapping with each
vibration of the cruise jet motor’, Szymanski later recalled. ‘I noticed there
were no ailerons, and also that on the front of the bomb was a silly little
propeller. It looked ridiculous. I decided to try to tip the “Doodlebug” up
with my wingtip. As soon as I put my port wing under the “Doodlebug’s”
wing it started lifting. I let it straighten itself out, then I put enough of
the front part of my wingtip under its wingtip, taking care to keep my
aileron out of the way, and then by a sharp bank to starboard I hit it with
the port wingtip.’ However, this failed to topple the bomb.
‘I then tried a slightly different manoeuvre, hitting it very hard with my
wingtip as I went up like in a loop. To my dismay the “Doodlebug”
continued to fly on perfectly straight. Then I realised that the engine
was now underneath. I had turned it upside down! I could see that it was
gradually going into a dive, and then down it went.’

Szymanski also recounted that he only managed to catch up with the V-1 after damaging it with gunfire, which slowed it down, before his ammo ran out. From what I've read elsewhere, this seem plausible because the V1 engine could take machine gun hits and still kept operating.
Somewhat similar story by Flt Sgt Paul Leva of No 350 Sqn (not entirely sure what aircraft they flew, but it seems they were  Spitfire XIV); he damaged a V-1 with gunfire, then tipped it over when his ammon ran out. He recounts that he could only catch up after damaging it, and he wondered how the thing still flew with numerous holes in its wings:

‘I climbed as quickly as possible to a nice comfortable altitude from
where, in principle, I would be able to dive on any bomb passing
underneath me. Soon after the familiar voice of the controller came on,
telling me that a flying bomb had been spotted on the radar, and he gave
me its position and heading. Soon, with my speed dropping after levelling
off, I could see that the distance separating us had not diminished. Indeed,
it had even begun to increase. Utterly disappointed, I nevertheless opened
fire, aiming high to compensate for the distance. I had the happy surprise
to see some impacts and bits flying off the wings.
‘I fired burst after burst, damaging the bomb still more. Although no vital
part was hit, its speed diminished and it entered into a shallow dive. My
hopes were soaring again. I was now approaching my target so fast that I had
to throttle back. Ready for the kill, I positioned myself at what I estimated
was the right distance. I depressed the trigger, but instead of the staccato of
firing bullets I heard only the sound of escaping compressed air.
‘It was then that I suddenly remembered the briefing we had received
about sending V1s out of control by tipping up one wing. Adjusting the
throttle, I eased myself forward until I came abreast of the bomb. What
a sight at close range! The wings were so ragged with the impacts of bullets
that I wondered how it could still fly almost straight and level. Positioning
myself slightly underneath, I placed my starboard wingtip under the port
wing of the bomb. I came up slowly, made contact with it as softly as I
could and then moved the stick violently back and to the left. This made
me enter a steep climbing turn and I lost sight of the bomb. As I continued
turning fast through 360 degrees I saw it well below me, going down
steeply, hitting the ground with a blinding flash.’

So, yeah, it seems pretty clear that the technique, despite being taught after the early couple of successes, was generally used only as a last resort. And V-1s already damaged by gunfire seem to have been a somewhat sizeable proportion of such tipping kills.

This answer has an amazing amount of information in it. I am surprised by the number of different types that accomplished this maneuver. I had halfway convinced myself that this had only occurred a couple times and the story was continuously repeated.

Pic source
It apparently did really happen, described in the book V1 Flying Bomb Aces by Andrew Thomas, page 19:

It was on 23 June that one of No 91 Sqn's Australian pilots, Fly Off Ken Collier, executed a cool act when on a 'Diver' patrol in Spitfire XIV...

The page subsequently describes how Collier had several shots at the V1 without anything happening and him running out of ammo, and

By this time Ken was really swearing mad, and was determined to do or die. He therefore formated with it and with his wing tipped it over.

The book also describes the normal procedure for destroying V1's, which was not by wing-tipping it over, but shooting at it from a range of 200-250 yards. From further away the flight controls or systems would often be hit, causing the V1 to dive to the ground and the warhead explode on impact. Closer than 200 yards and mostly the warhead would blow up, with the aeroplane following closely and being in danger from the explosion.
Mosquitoes, Spitfires, Mustangs, Thunderbolts are bing mentioned as successful V1 killers (by gunfire), with the Tempest V being the most successful plane during the day, and the night fighter Mosquitoes during the night.

My father flew C-47s in Burma and often had Spitfire escorts.  If the air was smooth enough, the bored escort pilots would sometimes make a game of easing up to the Dakota's wing tip and trying to knock off the most outboard static wick with the Spit's wing tip.

It occurred on many occasions. Based on the book "Mister Brown" written by Jan Zumbach (https://en.wikipedia.org/wiki/Jan_Zumbach). I don't have the original French version, but I have the Polish translation. He mentions German countermeasures of placing small explosive charges under V1's wings to prevent tipping. Two pilots of No. 303 Squadron RAF, he commanded, died after these charges exploded on contact, which put an end to the tipping method.

www.historylearningsite.co.I’m  claims it’s true:

Fighter pilots also learned new tricks to destroy V1’s such as flying alongside the weapon and tipping over one of its wings, thus knocking it off course. Pilots also flew in front of a V1 so that it flew in the fighter plane’s slipstream. This was enough to unbalance the V1 so that it flew off course.

The questions isn't whether it happened, but about how common it was.

@vsz - Op asked whether it was common or happened only once.  If it happened only once this quote would say “One fighter pilot even managed to fly alongside...” But it instead says, “pilots [plural] learned new tricks...” strongly implying that it happened more than once.   But I admit this answer is not as good as others here.


text:
Why don't piston engines require as heat resistant materials as jet engines?
Piston engines often get away with using aluminium as their heat exposed material, while even the early jets required inconel to operate.
Is that because the combustion takes only, for example, half of the time for two stroke engines hence half the temperature?
Still compression ratio of a typical two stroke is twice higher than early jets, while the melting point of aluminium is more than twice lower so it doesn't add up, as if there was another reason for their higher heat durability

The hottest that gasoline gets in an internal combustion engine is approximately 700 C. Meanwhile, jet fuel reaches around 2000 C. Maybe I don’t understand the question, but it seems like a much hotter burning engine would require more heat resistant materials.

Most piston aviation engines are 4 stroke.  And what do you mean by "front line material"?  I believe most cylinder linings and exhaust manifolds are steel.

i don't think theres much difference in combustion temperature, as gasoline and kerosene have similiar adiabatic flame temperature. On top of that combustion in pistons is combustion of stochiometric mixture inlike in jets, and it's also isochoric as opposed to adiabatic as in jet engines, so the peak pressure and therefore temperature are much higher

But the duration is much, much less than continuous...

@Francis L.: I'm sorry. I wasn't just guessing about those temperatures. I verified my facts before posting. Jet engines produce fuel burn temperatures that are nearly 3 times higher than piston engines and it is continuous, just as Michael pointed out

@Aaron Holmes could you send me the link to the source?

@Francis L.: "Inside the typical commercial jet engine, the fuel burns in the combustion chamber at up to 2000 degrees Celsius."  -  Retrieved from: https://cs.stanford.edu/people/eroberts/courses/ww2/projects/jet-airplanes/how.html

@Francis L.: "Ignition of the fuel in the cylinders produces temperatures of 700°C or more."  -  Retrieved from https://en.m.wikiversity.org/wiki/Internal_combustion_engines

@Francis L.: "Some typical temperature values for passenger car pistons are [3466] : Center of piston crown (gasoline engine, port injection) 270–310°C. Piston crown bowl (gasoline engine, direct injection) 270–350°C. Bowl rim (diesel engine, direct injection) 350–400°C."  - Retrieved from https://dieselnet.com/tech/combustion_piston-cool.php

@AaronHolmes, 700C is comparable to - or slightly lower than - the typical exhaust gas temp of an aircraft piston engine. You'd hope that the engine extracted some work from the fuel, so this should be well below the flame temp. Listed here (https://en.wikipedia.org/wiki/Adiabatic_flame_temperature) is an adiabatic temp of ~2000C for gasoline flame in air. The temp at constant volume (assuming burning much faster than timescale of piston movement) would be higher, since no work is done (until the piston moves outward in the cylinder).

@Foster Boondoggle: I know that what you're saying is true. And yet, when I look for typical cylinder temperatures during ignition phase, the temps are always much lower than that. Perhaps the discrepancy lies in the fact that the exhaust system receives constant heat input from all cylinders, while a cylinder in a 4-stroke piston engine only experiences split-second exothermic reactions one quarter of the time.

Lubrication.
The low wear of piston engines is due to an oil film between piston rings and liner, so both never get into immediate contact. This perfectly thin oil film left on the liner bore by the oil control ring will flash off if temperatures exceed 180° - 200°C, and the piston rings will scuff. This need for lubrication is the weak link which makes improving heat resistance for piston engine materials redundant.
Lubrication in turn requires sufficient cooling which is possible because combustion in reciprocating engines is a discontinuous process with only the end of one and the beginning of the next stroke producing compression and combustion heat while the others allow to cool down the cylinder head and walls.
Contrast this to jet engines. Continous combustion produces much higher thermal loads so any attempt at sealing the compressor and turbine disks against the outer casing is futile. Only the development of heat-resistant alloys has made this possible: Look at how the improving materials allowed increasing compression ratios. Modern turbofan engines reach compression ratios of up to 50:1 while diesels are stuck at around 20:1 and gasoline engines at around 10:1. Aviation pistons with superchargers raise that to levels similar to modern turbofans: The Wright R-3350 turbo-compound engine supercharger increased pressure by 6.5:1 with the pistons compressing air again by 6.85:1 for a total of 45:1. The highest compression ratios were achieved with super- or turbocharged diesels. The Jumo 205D supercharger achieved a compression ratio of 8.85:1 and the engine another 17:1 for a total of 150:1, but in all cases needing intercoolers to keep air temperature low enough and using waste gates or staged chargers to only reach the top compression ratios close to their critical altitude!
Since decades we can read about "adiabatic engines", piston engines using heat-resistant ceramics and doing away with the whole cooling system. Their implementation has so far not happened because the wear between piston and liner has been unacceptable (among a host of other problems, like manufacturing cost and brittleness).

Though we have wandered a bit from "why don't piston engines require as heat resistant materials as jet engines" to "much better to build them with heat resistant materials, and make them turbines", this has been very informative.  Would you consider including the turbo compresser in piston engines when comparing with jets?  I have read that jets get 1:50 "at the top of their climb, going to cruise" (in much thinner air).  How would this compare with a supercharged piston engine?

@RobertDiGiovanni: Yes, good point. Especially the turbo-compound engines look thermodynamically like jets with a discontinuous combustion process. However, super- or turbocharging rarely went above 1.5:1 (at sea level), on engines which sported 8:1or less initially. Near the engine's critical altitude this could reach 8:1, but total pressure and cylinder wall heating was still restricted.

Piston engines are designed to produce mechanical energy, torque, which is used to move a propulsion unit. In aircraft, it is the propeller, in cars, the wheel.  There may or may not be a transmission. Heat produced is waste (after expansion to drive the piston) and is removed by a cooling system and exhaust.  The enclosed piston is more efficient than a jet at producing mechanical energy per unit of fuel, just as the cannon is more efficient than the recoiless rifle at propelling a shell.  With pistons, combustion of fuel is not constant, allowing heat sinks such as air and water flow, to remove the heat before temperatures get too hot.
Good conductors of heat, such as aluminum, brass, and chromium steel, work well with these applications, but high fuel burn rates can lead to overheating.  Piston engines are RPM limited, and simply cannot produce enough power as ...
Jets. Capable of producing much more thrust by using heat resistant metals and running at much higher RPM.  Here the "internal combustion" is replaced with less efficient but lighter jet engines.  Jets have an "Ace in the Hole", the ability to maintain thrust at higher altitudes, where greater TAS off-sets lower propulsion efficiency.
This is why the 747 has double the ton miles/per gallon fuel burned efficiency compared with even the mightiest of the piston engined transports.  Lower and slower, piston props are better.

Excellent point about cooling...

It is indeed about cooling. But the main difference is about ability to carry the heat out. Pistons can be cooled from outside easily, but the core parts of a turbine engine are surrounded by the hot gas and only cooling medium available for them is the compressed air from before combustors, which is still quite hot from the compression.

The RPM is not too relevant. Higher RPM does increase the power/weight ratio, and that means needing to take more heat from the same mass of metal, but since the principles are different, the numbers are not directly comparable.

@Jan Hudec RPM is relevant as far as friction (lubrication) and heat produced in an internal combustion engine.  But I would suggest high RPM $electrics$ for future applications. "Core parts of a turbine engine are surrounded by the hot gas and (the) only cooling medium available for them is the compressed air from the combustors" is actually an excellent answer to this question!

@RobertDiGiovanni, piston engines generally operate at lower RPM in the same power class (race car (F1) engines go to 18,000 or 19,000 RPM these days, but then a PT6, which is in the comparable power class goes to ~45,000 Nc and ~30,000 N1), but they have much more friction surface. In piston engine it is all the piston rings plus the rod bearings which are usually sliding, while in turbine all bearings are roller ones, so there are just the labyrinth seals that come anywhere close to creating friction. But it's still minor issue compared to the combustion heat for both.

Jet engines are also not actually better at maintaining thrust at higher altitudes — for both piston and jet engines the thrust is proportional to air density (but above some density, also both, become limited by temperature and/or total pressure). What they are actually better is at maintaining thrust at higher velocity, to the point they are more efficient at higher speeds. The higher altitude is just to maintain more reasonable dynamic pressure.

Modern turbofans are more efficient at producing power than gasoline-powered piston engines, not less. You can see it by checking the SFC of aeroderivative power turbines vs piston engine BSFC. Jet engine cores do about 200 g/kWh, cars 220-270. The O-360 is about 260.

@Jan Hudec Yes, thrust at higher velocity.  The lower RPM propeller cannot maintain its optimal AOA, whereas the higher RPM fan can, there for the jet can go to a higher TAS (at higher altitude) to get the same IAS.

@Therac though way off subject, you have a very good point, and Peter's info also shows the huge improvement in modern turbines. Noticed a trend in size and efficiency (so did Boeing).  Very informative, thanks!

@RobertDiGiovanni, it's not as much about propellers not being able to do something – propellers keep their propulsive efficiency (power applied to the aircraft to power delivered from the engine) almost constant all the way until the tips approach speed of sound – but about turbines operating with decreasing delta-v, utilizing pressure recovery to maintain enough mass flow.

@Jan Hudec Yes, and higher RPM props will be more efficient at higher speeds, but the piston engine just can't generate enough power to turn them without overheating.  So the discussion becomes application driven.  A jet can go to a higher TAS, which makes the airplane more efficient at the same IAS (that the wing optimal AOA flys at, unless it approaches Mach limit).  But the larger turbines seem to be doing great with SFC, down to 200g/kW/h.  The smaller PT-6 is around 350g/kW/h.

@RobertDiGiovanni, the airplane is less efficient at higher altitude, though just a little bit. At the same IAS the drag increases slightly with altitude due to the cold temperature, and so does work to get the plane from A to B, because work is force (drag) times distance. What is more efficient is the jet engine. Both piston and turbine engines are a bit more efficient at altitude due to the lower temperature, but jet engines add massive improvement of efficiency with velocity (TAS). They do more work per unit of fuel at higher speed.

@Jan Hudec. Well 2 projects for us then:  1.  Put a prop on a GE 90.  Fly a B-52 at 50,000 feet off vs a 787 at 30,000 feet (they are almost identical in weight and demensions).

Piston engines do not get as hot ( the metallic components) ; the exhaust gases are hot but not nearly as hot as the first stage of the turbine. Remember the cylinders and pistons are made of aluminum which melts at 1000 +/- F and is pretty useless at 600 F. The first turbine stage gases were up to 1800 F when I last read about them . The high performance turbine blades have small axial holes for cooling air to flow through them. And turbines are always hot while a piston engine only reaches its highest temperatures on one of 4 strokes. The exhaust valves are made of high temperature alloys .

I'm not claiming they're as hot as jet combustion products, but piston EGTs are regularly in the 1500+ deg F range. The aluminium portions of the cylinders and pistons certainly remain at much lower temperatures.

Yes, exhaust manifolds will be red, about 1400 + F running under load. But pistons , heads don;t get so hot during the limited time of the combustion stroke.


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What are the least powerful airplanes that ever flew?
What manned airplanes have achieved flight with the least powerful engines (no gliders!), and what was their top speed? Im sure I've heard of an airplane with an 8hp engine capable of exceeding 120 MPH airspeed

The one you're probably remembering was the original Rutan Quickie -- it had an 18 HP Onan industrial engine, and top speed of just about 120 mph.  With 75 mpg at cruise.

actually, I found it, it was storch III build by Lippisch

For reference, I believe the Wright Brothers' first plane was about 12HP and a 20mph max.

The only Storch I can find was 240hp. It was the stall speed of 31mph that was exceptional, as well as landing in 100' and taking off in 250'. The bit about rescuing Mussolini was interesting.  Fieseler Fi 156 Storch

https://youtu.be/s__sKI0GSRw storch 3 is referenced within first five minutes

The Lippisch Storch series are a currently offered tailless type ultralight, not connected in any way with the Feisler STOL offering of the 1930s and 1940s.

@ZeissIkon "Onan industrial engine"  A childish part of me is sad that the Onan Industrial Engine company didn't build Wankel engines.

@Graham I'm so with you on this one! And, if they would have, they absolutely should have been used on Fokkers.

@jpe61 Old classic.. http://www.circlecity.co.uk/text_jokes/spitfire-pilot.php

Matthew Bacon Sellers II built a quadruplane and flew it from ca 1909 to 1912, progressively reducing the engine power to see how low he could get it.He managed to fly on around 5 hp. 120 mph on 8 hp is not credible. But is man-powered flight allowed?

@jpe61 Legend! :)

There are a number of human-powered aircraft (list here).  For the Gossamer Albatross, we have

In still air, the required power was on the order of 300 W (0.40 hp),
  though even mild turbulence made this figure rise rapidly.

As far as top speed:

Allen completed the 22.2 mi (35.7 km) crossing in 2 hours and 49
  minutes, achieving a top speed of 18 mph (29 km/h) and an average
  altitude of 5 ft (1.5 m)

was he in ground effect during his flight?

At 5 ft altitude, almost certainly.  Looked for one that wasn't in ground effect, but can't find one quickly.

Seems reasonable - a top-notch pro cyclist can average ~300 watts for a 6 hour race, and average 400-500 watts for a climb of under 30 minutes.  Us mere humans might be half that.   745 W is one horsepower so ballpark 1/3 HP to 1/2 HP.  Its hard to measure accurately biomechanical output because of the many variables,

@EugeneStyer The original MacReady prize required the Gossamer Condor to get out of ground effect to win -- IIRC, they had to climb to 30 m at least once while negotiating the figure-8 flight course.  They didn't stay there long, because it was just about all the pilot/engine could do to get that high for a few seconds in dead calm dawn conditions.

See Ground-effect vehicle and Ground effect (aerodynamics) on Wikipedia

Sailplanes employ zero horsepower by conventional reckoning, though an alternative definition can be proposed using the component of the aircraft's weight that acts parallel to the airspeed vector as the thrust-like force.  Anyway, the world records for sailplane flight appear to include 2191 km as greatest free distance along a course involving three or fewer turnpoints, and 22657 m as maximum absolute altitude.   Source: fai.org/page/igc-records
OK, I thought the original question said "aircraft"; I now see "airplane".  Does "airplane" always exclude "glider"?  Originally, the "plane" in "airplane" referred to the "planing" action of the wing surface, and had nothing to do with the presence or absence of a motor, though there may be no specific examples of the word historically being used in reference to gliders.
Didn't see the "no gliders!" when I created this answer-- sorry!  Some self-launching powered hang gliders and powered paragliders/ paramotors intended for soaring flight must be close contenders for the answer to your question, as some of them have rather weak engines and are barely able to climb in the absence of an updraft despite overall light weight.

"are barely able to climb"-- in the absence of an updraft, of course.

Some of the earliest powered hang gliders, and again paramotors, had as little as 6 hp -- this level of underpower has fallen out of favor due to safety considerations.  Just takes too darned long to get to safe altitude (i.e. high enough to recover from an upset).

yes, "and paramotors" would be a good addition to last sentence in answer

But clearly, human-powered aircraft are the best contenders for answers to the question.

Oh, yes.  The lowest powered successful powered ultralight or paramotor had close to twenty times the long-run power of a top tier bicyclist.

The No Gliders was always there.  You can click the "edited yesterday" link below the question to see the revision history.

I'm sure it was, I just didn't see it, sorry.  Should edit to remove "was that added..."

Gliders use 'gravitational power'. You multiply the weight in newtons by the sink speed in m/s and get the gravitational power in watt...

This is of course much higher than the human-powered Gossamer Albatross in Eugene Styer's answer, but the Wright Flyer, "the first powered, heavier-than-air machine to achieve controlled, sustained flight with a pilot aboard", used a

12 horsepower gasoline engine

which is a bit under 9 kW.
The longest of its 4 flights was 260 m, and it was apparently so light that "a heavy gust picked up the Flyer and tumbled it end over end, damaging it beyond any hope of quick repair".
Later versions used more powerful engines.

Even though at 4 x 13kW its max rated power (70hp) is higher than the other answers, Solar Impulse flew almost 5 thousand miles non-stop during close to 5 days. It did not typically fly anywhere close to its max rated power in order to conserve energy that it would have to spend during the night.
Wikipedia lists the specs as:

Maximum speed: 140 km/h (87 mph)
Cruise speed: 90 km/h (56 mph) 60
  km/h (37 mph) at night to save power
Service ceiling: 8,500 m (27,900
  ft) with a maximum altitude of 12,000 metres (39,000 ft)

The lightest remote control airplane I could find is 0.225g (0.01oz).
Wing span 71 mm.
Length 70 mm. (about 3")
I estimate it used a few hundredths of a watt.
https://www.rcgroups.com/forums/showpost.php?p=11048403&postcount=1
That was back in 2008 though, so they've probably done better since then.

"Manned airplane" -- this isn't that.

@ZeissIkon  just gotta find a really small person. https://en.wikipedia.org/wiki/Downsizing_(film)

Yeeeeah-no.  Maybe on the SF&F stack, or Worldbuilding.

Oops, I noticed that the title didn't specify 'manned' but missed it in the text.

@CarlWitthoft "Honey, I shrunk the pilot?"

It seems as if they can teach rats to drive cars (https://www.caranddriver.com/news/a29563243/rats-driving-cars-stress-study/) they can teach them to fly airplanes.  Should reduce ticket prices.  Just don't let any cats get in the cockpit.

Ah, good thinking though!

Well, I know that some air taxi services are planning to use UAVs for carrying passengers.

The twin engined Columbian Cri cri might be a contender: each of the single cylinder piston engines were about 15hp. Cruising speed is 190km/h.
Wikipedia article


text:
Why were trim tabs invented?
I understand the need for a trimming mechanism, but why use a secondary control surface placed on top of the first, when you could just adjust the resting position of the aileron/elevator/rudder itself?

You have to have some way of holding the surface at other than where it wants to naturally trail in the airstream.
Your options:

Hold it yourself with control pressure. Becomes a pain after a while.  Some very simple homebuilt designs don't have any pitch trimming device and they have only one trim speed where they will fly hands off, the trim speed more or less controlled by stabilizer incidence; to fly faster or slower you have to hold continuous pressure on the stick.
Put a spring in the control circuit, a bungee, where the spring can be tensioned with a trimming device, usually a lever or screw device with a friction lock of some sort, to hold the continuous control force in the control circuit so you don't have to.  Gliders do this, and some light airplanes and homebuilts for pitch.  Works really well for aileron and rudder trim and lots of GA use bungees for the rudder.
For pitch trim, make the horizontal stabilizer incidence adjustable from the cockpit.  The elevator itself just trails when hands off the controls.  The fabric Pipers and Cessna 180/185 family do this, but you have a screw jack device and a pivoting stab attachment etc., so it's a bit complicated.  The fabric Pipers also combine the screw jack with a bungee interconnecting the elevator and stab so the stab applies a bit of elevator force when it moves, giving a bit more overall authority.
Use an external servo device to apply a force to the surface.  You have the air blowing past, so why not stick another moveable surface at the trailing edge that can move independently of the main surface, and apply an aerodynamic servo force.  A bit more drag than a bungee trim or adjustable stabilizer, but much more sensitive and versatile.  Much lighter and less complex than an adjustable stabilizer.

Probably 80-90% of the airplanes with manually operated flight controls use trim tabs for pitch trim because it's just the most elegant solution taking in all the considerations - light, simple, effective.  Tabs are less common for aileron or rudder until you get up to  airplanes over 5000lbs or so and you tend to see bungees or nothing, or little fixed bent metal tabs that only trim out to a single flight condition.

Still, it takes force to deflect the trim tab against the wind, and I don't see how is it less than the force needed to deflect the primary control surface itself. Since the force required to deflect = the force exerted by deflected trim tab. EDIT: Actually it makes sense now: the length of, let's say, the elevator is the lever on which the force coming from the trim tab acts on the elevator, so this gives a mechanical advantage requiring less force

They are pretty versatile.  You can gear the tab to also respond to the surface movement, and can use it to boost your controls.  That's a servo/trim tab, or just a servo tab if it's geared but not adjustable.  You can also gear it to go the OTHER way, and work against the surface and artificially increase the resistance with surface movement, an anti-servo tab.  Airplanes with all-flying tails like Cessna Cardinals and Cherokees get all of their static pitch stability from an anti-servo tab, with an adjustable neutral function for pitch trim.


text:
Why can't ramjet diffusers be stacked to achieve a good overall pressure ratio?
The problem with ramjets is that they only gain efficiency similiar to turbojets at speeds above mach 2. However, ramjets can operate at speeds of a few hundred km/h. Why therefore cant ramjet diffusers be stacked, analogously to compressor stages in a turbojets to achive a high compression ratio and efficiency at low speeds?

A diffuser is simply an expanding piece of pipe that converts kinetic energy (i.e. velocity) to pressure energy (i.e. pressure) according to Bernoulli's principle (which is just a formulation of the law of conservation of energy for fluid flow).
The air enters the diffuser with the velocity corresponding to the speed of the aircraft, and must still retain some speed through the combustion chamber. So only the speed above that can be converted to pressure. Few hundred km/s is already more than the speed in the combustion chamber, so there is some pressure gain, and the engine starts to work at all, but only above M2 the pressure gain becomes comparable to what a trubo-compressor can provide.
Also diffuser is simply an expanding piece of pipe and stacking two expanding pieces of pipe behind each other is still just an expanding piece of pipe. And two expanding pieces of pipe with a narrowing piece in between is no better than one of them, because the pressure only depends on the initial and final cross-section and not on the shape in between.

The analogy is that ram pressure REPLACES the compressor in a ramjet.  No need to suck in air and compress it.  You just need an orifice to match your desired speed and a slightly larger convergent/divergent nozzle at exhaust for thrust.  Supersonic ram jets also feature an "inlet spike" to slow down incoming air to subsonic speed.
The Air Force BOMARC B used solid fuel boosters to reach sufficient speed, and 87 octane gasoline Marquardt ramjets as sustainers.  It flew at around Mach 2.5 as an unmanned
surface to air interceptor.


text:
How are NOTAMs distributed and received?
Can you only receive NOTAMs via teleprinter? If so, how do you receive them in flight in a plane without a printer? Also, how is a NOTAM generated? I'm trying to read an Oxford Air Law book and it is so hard to understand.

There are multiple ways, at least in the UK.  On the internet for example.

Related questions: here, here, here, here. It might also help to mention which country you're interested in; sometimes local procedures can be different.

FWIW, in the 1990s at the two 747 carriers I worked for, printed NOTAMs were delivered to the captain with the printed flight plan. Usually they had been printed by a dot matrix printer attached to a computer. However, at some out stations they were fax copies, and sometimes, especially in third world countries they were printed by teletypes.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
Will the new Part 23 changes reduce the cost of avionics for existing aircraft?
Will the part 23 changes coming in August help to reduce the cost of IFR GPS avionics? I'm looking to upgrade the IFR avionics in a 1978 Cessna 182.

Part 23 rewrite is mostly for the performance based certification standards on aircraft. This may have some effect on avionics, but if you are flying around in a certified airframe you'll need TSO'd avionics. It may help bring prices down a little bit, but the $1000 or so you might save on "cheap" avionics is, in my opinion, not worth the savings over name-brand stuff from Garmin.

@RonBeyer You can buy the same G5 from Garmin for experimental aircraft for $1,000 less than certificated aircraft. And the reason we have the G5 is because "cheap" avionics Dynon proved that the technology works in experimental aircraft and the FAA was persuaded to TSO it. A performance based standard is long overdue for avionics, and the FAA is taking baby steps toward that goal.

@JScarry The TSO/Non-TSO isn't going to change. Not saying that a performance based standard isn't where we should be.

a link to the "new part 23 changes" would help in making this question a bit more specific and not mixing it up next time there will be changes.

Unfortunately not: the purpose of the part 23 rewrite is to make it easier to certify a new aircraft type (not individual avionics or other systems) and it doesn't apply to existing aircraft. AOPA has a good overview, including this (emphasis in the original):

10. Are the performance-based standards applicable to all aircraft?
No.  The new process of using performance-based standards is limited
  to new airplanes certified under Part 23, which would include
  airplanes with a maximum passenger-seating configuration of 19 or
  less, and a maximum certificated takeoff weight of 19,000 pounds or
  less.

To address the cost point directly, in theory if the part 23 rewrite creates additional demand for new aircraft, and if those aircraft use avionics that can also be retrofitted to existing aircraft, then the increased demand might reduce prices. But that's highly speculative.

I would say that the answer is a qualified 'yes.'  The revised rules do not modify the need for type certificates nor does it affect the certificates of any existing certificated aircraft.  So the major benefit will be when attempting to certificate a new aircraft design.
But the new rules can be applied to a modification to an existing type certificate.  In fact the FAA normally insists that when you make a major modification to an existing design, the changes must be compliant with the current rules.  This has historically been a burden because the rules kept getting tighter as time went along.
With the revised Part 23, the new rules can be used as the certification basis for an STC to an aircraft.  The primary issue is that this isn't a cheap process regardless of the underlying certification basis. But EAA has been working to obtain STCs that can be used just for this purpose.  In fact, they were able to obtain an STC for the installation of Dynon EFIS-D10A or EFIS-D100 as replacement for the primary attitude indicator. It should be pointed out that the Dynon equipment does not have a TSO or a PMA, but is  verified against the recently developed ASTM 3153-15, Standard Specification for Verification of Avionics Systems.
EAA's STC has an approved model list.   From EAA's site:

Currently, the Approved Model List (AML) includes the following
  aircraft series (including all variants on each type certificate):
Beechcraft Bonanza/Debonair, Beechcraft Musketeer/Sundowner/Sierra,
  Beechcraft Skipper
Cessna 150, Cessna 152, Cessna 170 Cessna 172, Cessna 175, Cessna 177,
  Cessna 177RG, Cessna 180/185, Cessna 182, Cessna 205/206/207, Cessna
  210
Grumman AA-1/AA-5
Maule M-4/5/6/7
Mooney M20
Piper PA-24, Piper PA-28, Piper PA-32, Piper PA-38

EAA members can purchase the STC from EAA for $100, allowing the Dynon equipment to be installed in any of the AML aircraft types.
Hopefully the use of this type of a process will expand and provide for more options at lower price points than in the past.  While it isn't a panacea, it will help.


text:
How can I convert a Georgian CPL to an EASA frozen ATPL?
I am interested in how I can convert my CPL issued by Georgia to a frozen ATPL? Georgia is not a member of EASA.

Welcome to aviation.SE! I edited your question a little, I hope it's correct but you can always edit it again yourself if I made a mistake.

Does your Georgian license comply with ICAO standards?


text:
Can I pass the EASA ATPL exams by studying FAA materials?
If I have the theoretical knowledge required for an FAA ATP, could I use that knowledge to pass the EASA ATPL exams?

Can you clarify what you mean by "pass" the exam? In some languages that means to take or sit the exam; in others it means to successfully complete the exam. I'm not sure if you're asking if a) knowing the FAA material is good enough to answer the EASA questions, or b) you're allowed to sit an EASA exam after doing an FAA study course. My edit assumes (a) but I might be wrong.

Someone will surely come in with an answer, but I recall that you cannot sit the EASA exams without first completing an EASA approved course. I doubt the syllabuses are all dramatically different but I think EASA goes into a lot more detail than the FAA.

@Pondlife What variety of English uses the word "pass"  to mean "take or sit an exam (without necessarily obtaining a favorable grade or mark)"? I don't think I've ever heard that usage.

@TannerSwett Non-English speakers often confuse "taking" and "passing" exams and I thought it was worth clarifying here; the question is very short so there's no context to confirm the meaning. I'm not saying that Ucha is using it incorrectly, but it's a very common source of confusion.

If you are asking about knowledge - you can use some points, but generally FAA ATP knowledge is insufficient. EASA has 14 subjects, and requires much stronger theoretical background. Just go through the EASA learning objectives and you will see it.
Moreover, there are some differences in Air Law (EASA exam pretends to be ICAO-based only, but in fact it asks about European regulations as well), IFR and VFR communications, and even units of measurement (for example, EASA has some questions where you pick altitudes in metres, not in feet).
And, finally, you cannot even sit the exams without taking a theoretical course in some EASA-approved flight school. That means that you usually need a course completion certificate before going to CAA. There is another way if you have 1500 hours under your belt, but that is a completely different story, and in that case sometimes you don't need to sit the exams at all.

There are lot of online question banks which helps you clear atpl. Just work hard. https://www.hashtrek.com/groups/atpl-question-bank#latest


text:
How can I convert an ICAO (Georgian) CPL to an EASA CPL?
I have an ICAO CPL issued by Georgia. The Republic of Georgia is not a member of EASA, but it is included in the category Countries with whom EASA has a Working Arrangement. I have 150 flight hours, 100 in an Aeroprakt A-22 and 50 in a Cessna.
In the future I am going to study the EASA ATPL theory course in an EASA member state and afterwards convert my ICAO CPL to an EASA CPL. Please let me know what kind of procedures do I need to follow? (In addition to sitting the exams in 14 subjects). I know that I also need to sit flight exams and I need further information about that. Is it worth converting my license to EASA or not?

This seems to be a duplicate of your previous question. Usually, you should edit a question to add more information instead of opening a new one, but you've given more useful information here, so I've voted to close the older question.

"How to" - the best option I'd bet is contacting the local authority (mobility / transport government entity) and they may give you the best answer.


text:
When did pilots start making announcements to passengers?
It's common practice now, and maybe required for most airlines. But I don't think that earlier pilots made the same announcements that they do today. When did pilots start making in-flight announcements to passengers? And all the announcements sound the same, is there a standard format for making announcements?

What announcements are you talking about? The safety briefing or notifications about the flight/other fun stuff?

Which “pilots announcements” do you have in mind? The safety briefing is given by cabin crew, or sometimes pre-recorded, but it is never by pilots.

Other fun staff, but if it's never by pilots, I'm sorry I didn't now

We had a very similar question already, although it has no answer yet

I edited your question to hopefully make it clearer but if I misunderstood what you're asking you can simply roll back my edit, or edit it again yourself.

Thank you @Pondlife, I have some problems with English and try to deal with it, I think my questions are misunderstand because of this

This varies by jurisdiction and air frame in question as the brief is usually specific to the air frame. Ill answer for the FAA.
What the pilot needs to say (by law) is governed by (follow link for full text),

§ 121.571 Briefing passengers before takeoff.
(a) Each certificate holder operating a passenger-carrying airplane shall
  insure that all passengers are orally briefed by the appropriate
  crewmember as follows:
(1) Before each takeoff, on each of the following:
(i)Smoking. Each passenger shall be briefed on when, where, and under
  what conditions smoking is prohibited including...
(ii) The location of emergency exits.
(iii) The use of safety belts...
(iv) The location and use of any required emergency flotation means.
(v) On operations that do not use a flight attendant, the following
  additional information:
(A) The placement of seat backs in an upright position before takeoff
  and landing.
(B) Location of survival equipment.
(C) If the flight involves operations above 12,000 MSL, the normal and
  emergency use of oxygen.
(D) Location and operation of fire extinguisher.
(2) After each takeoff, immediately before or immediately after
  turning the seat belt sign off, an announcement shall be made that
  passengers should keep their seat belts fastened, while seated, even
  when the seat belt sign is off.
....

FAR 121.571 comes off Doc. No. 2033, 30 FR 3206, Mar. 9, 1965 and was presumably added to the regulations in 1965.
Yes there is a standard. You can find more details in this Advisory Circular AC121-24C which covers what the oral brief needs to contain in more depth and outlines the standards. This AC has been in place since 2003 and replaces AC121-24B which took effect in 1999 I cant find the full text of that one so Im not sure what it replaced (presumably AC121-24A).
There is no direct regulation in the FAR's (or current AC) that require announcing things like route, altitude or temp.

I'm not sure this answers the question fully because the OP is asking about announcements made by the pilot. Your answer is about required passenger briefings, which are typically done by the cabin crew ("the appropriate crewmember"). But the question is a bit vague, so I may be wrong.

I would agree, however it is a bit vague and the regulations/AC cover pilot and crew announcements which look like they are lumped together when it comes to regulations.

Again, that's nothing pilot has to say. Note that the quote says “crewmember”—when there is cabin crew, they brief passengers, not pilots. Pilot only does this in very small aircarft that don't have cabin crew.


text:
How to navigate at night after a complete navigation systems failure?
How do pilots deal with incorrect navigation readings at night? How do they know where to fly?

At low altitude do they have to know the shapes of cities and villages?
At high altitude and speeds over an ocean, what is the procedure?

It's perfectly legal and safe to operate VFR at night - ie, no navigational equipment apart from a map, compass and a watch!

@Dan That's true in the US, but there are other countries that don't allow VFR at night, e.g. Mexico.

This is no different then flying during daylight. Your first choice is GPS ( for most people :)). Failing that you fall back on radio navigation ( VOR ). Failing that you fall back on your charts and NavLog. NavLog is essentially pilotage and you rely upon your airplane speed and direction and checking waypoints along the way. If you chose your waypoints carefully ( on the ground when creating your NavLog, I use airports so that I can land of them in case of an emergency) and you know how they look like from air ( I take satellite photos ), assuming that your waypoints are 10-15 minutes apart and you took care of wind speed and direction in your navlog, there is really a minimal chance of getting lost.
Ocean is a different matter. Maybe you will need to look at stars or something. In VFR we are told not to fly over the water, day or night.

I forgot to mention that you can also call up ATC and ask for directions.

Not to be relied on in say the middle of Nevada (at GA altitudes, anyway).


text:
What is the average total flight time to get a private pilot license in SF bayarea, CA?
I've heard that to get a private pilot license in bay-area, CA, San francisco is a little bit harder than other place, because of difficult and busy airspace, ATC, air traffic and some reason.
Do you have any idea what is the average total flight time to get a PPL it in bay area? Thank you.

I mean total flight time. Thank you!

The SF Bay Area is more complex than some other airspaces. This does not mean that it will require more time, you just need to prepare for it.
I live on the SF peninsula and while I do not have a broad survey, I trained at the same time as two friends so I have three datapoints. I got my license with 41hrs training, one friend was under 50hrs and the other somewhere in the 60s. I doubt this is above the national average. This was some years ago, but the sky was already full. I guess now it is more full.
I picked my own instructor and did not let him teach me anything that I could learn from a book or on my own. We had every session topic planned ahead of time and I made sure I was well prepared. Far more important than where you train is your own attitude and diligence in preparation.

Nice example, thanks for your information

@kaizen As a flight instructor, I feel the need to comment here... getting your certificate is a LOT of new information. I, too, studied like crazy and was ready in the low 40s. However, in the same way Pilothead cautions against teaching extra, I caution against learning the legal minimums to get by. Like with everything, some instructors are more thorough and a little extra of their time is worth its weight in gold! Pick an excellent, thorough instructor. Even if it takes a little longer with them, you’ll be way better off than rushing with someone who just wants to get you “through.”

@Pugz I in no way cautioned against learning extra or recommended stopping at legal minimums. I simply said that there are multiple sources of information and I made best use of each source. I did not wait to be spoon fed by an instructor. By the time I took my PPL checkride I was well into training for my instrument endorsement. My friend with 60+ hours failed his first ride and had to do it again. Flight time does not equal proficiency.

I got my license in 1975 and the school I used had a syllabus so every instructor knew what we needed to do. Each lesson was checked pass or fail. I only remember having to re-do two lessons (lazy-8s and cross wind landings). The only ground school I needed was xcountry reviews. My primary instructor used the 10min to and from the practice area to expound on personal wisdom - well worth it (that's where I learned the stuff that later saved my life several times).

A big factor is the  airport you operate out of. How far do have to taxi to/from the runway. I've been on fields where 0.5 hr taxi time was common.  How far is the airport from the training area?  You can make some use of that time, but it still adds up. A smaller uncontrolled field will normally allow more touch and goes than a larger field -- the longer the runway, the bigger the pattern. Mix in some large a/c and you can kill time waiting for wake turbulence.

As a CFI, I can teach you to take off, fly around and land (with reasonable crosswinds) in 5 hours.  The rest of the time is spent on rules/regulations and judgement experience.

I learned on a 1659' paved field, very narrow, 45', with tall trees on the southeast end and a fence to keep you out of the road on the northeast end.  Had to be quite good at landing before you could solo.  On the other hand, everyplace else seems excessively long now!  Found  a photo on flicker

https://www.flickr.com/photos/masstravel/15548278201/in/photolist-pFWZq6-p1jpoe-HpPFmV-69uZ3P-mL3NSU-9vmQ4S-iPskmF-7CqP8h-jCDpF6-4vJ53S-aMP8W-XLA2Vp-h1DA5Y-qFbcXM-GSWvuP-fc9js4-icoDE7-6zT59E-aDTRDG-qtaQsX-K9U4mm-25zd59A-g8og8K-4qq2Kb-r5FCcU-25jbmxG-22BygR3-e2RuT-icowTy-icoXkX-qt7FTa-qEjhGy-festuN-icoX5r-9pfFiD-fed2Mc-e9dFoS-aMRbc-icoVCD-tKrU5N-d9NPfY-5zwD2J-jzj714-Ton7RM-d9NMN7-9xYwE4-21cye21-ZzM2oP-T2Pekz-HP995v

@Pilothead "Flight time does not equal proficiency." encourage me.

@Gerry That's true.

@CrossRoads Looks nice.


text:
Does the HAL LCH have fly-by-wire?
Is Automatic Flight Control System (AFCS) the same as Fly-by-wire?
If not what's the difference?

India’s Hindustan Aeronautics Limited (HAL) has carried out the first
  flight of a Light Combat Helicopter (LCH) using an indigenously
  developed Automatic Flight Control System (AFCS), the state-owned
  aerospace and defence company announced in a statement on 31 January.

Source:
https://www.janes.com/article/77568/hal-tests-indigenous-flight-control-system-for-light-combat-helicopter

AFCS is just a fancy word for autopilot.  Autopilots for helicopters are a bigger deal than on fixed wing airplanes, due to the difficulty in flying them, so it's no small thing to develop one.  In the end however it's still just a computer than can operate servos attached to the controls to move the mechanical controls in place of the pilot when it's turned on.
Fly By Wire is a whole different universe, where there is a computer(s) between the pilot and the flight controls, with no mechanical interface at all, and the pilot makes inputs that tell the computer what the pilot wants to do, and the computer decides how to move the surfaces.